1. Field of the Invention
The present invention relates to a radio communication method, a base station and a mobile station which can be used in the radio communication method.
Particularly, the present invention relates to a radio communication method using a CDMA (Code Division Multiple Access) method with the spread spectrum technique and having the cellular configuration, a base station and a mobile station which can be used in the radio communication method.
2. Description of the Related Art
Conventionally, a DS-CDMA (Direct Sequence CDMA) system in which a plurality of mobile stations performs radio communication using the same frequency band is known. In the DS-CDMA system, each mobile station is identified with spreading codes, and orthogonal codes such as Gold codes are used as the spreading codes.
In the DS-CDMA system, while a signal from a specific mobile station is despread in a receiver (a base station), an interference signal power from other mobile stations becomes “a transmission signal power of the other mobile stations/PG (Processing Gain)” on average. Particularly, a signal from each mobile station is influenced by an instantaneous variation, a short-term variation and a distance variation caused by each independent fading in the base station, under asynchronous environments in the uplink direction.
Therefore, the signal from each mobile station is satisfied with a required receiving quality in the base station, using a transmission power control in which SIR (Signal to Interference Power Ratio) is controlled to be constant in the base station.
In the conventional DS-CDMA system, however, there is a problem in that the spreading codes are not completely orthogonal, and the signal from a specific mobile station is influenced by the interference signal power from other mobile stations (a mutual correlation) under multi-path environments, even though the above transmission power control is performed completely, and the SIR is ensured to be constant in the base station.
In other words, there is a problem in that the interference signal power increases and the communication capacity per one cell is reduced, as the number of mobile stations performing the radio communication in the same frequency band increases under the multi-path environments in the conventional DS-CDMA system.
In order to solve the problems, an interference cancellation technique which can reduce the influence of the interference signal power from other mobile stations is known.
An adaptive antenna array diversity technique is known as the interference cancellation technique. The adaptive antenna array diversity technique realizes transmission and reception of directional beams by assigning adaptive weights to signals received in each of a plurality of antennas, and reduces the interference signal power from other mobile stations.
For example, a coherent adaptive antenna array diversity (CAAAD) receiving system is proposed, in the technical literature “Pilot symbol-assisted decision-directed coherent adaptive antenna array diversity for DS-CDMA mobile radio reverse link, IEICE Tans. Fundamentals, vol. E-80-A, pp. 2445–2454, December 1997. (S. Tanaka, M. Sawahashi and F. Adachi)”, as a type of the adaptive antenna array diversity technique.
Referring to FIG. 1, the CAAAD receiving system will be explained briefly.
First, a base station adopting the CAAAD receiving system receives signals transmitted from mobile stations at M antennas 1011 to 101M.
Second, the base station multiplies signals yk,m,l (m=1 to M) despread via antennas 101m, a RF radio unit 102 and matched filters 103m, by weighting factors (antenna weight values) w (w=w1 to wM) provided by a weighting factor controlling unit 104. The base station combines the multiplied signals in an adding unit 105.
The signals yk,m,l are acquired by despreading signals received over a path #1 with a mobile station #k at the antennas 101m (m=1 to M). The combined signal zk,l is represented by the following expression.
      z          k      ,      l        =            ∑              m        =        1            M        ⁢                  y                  k          ,          m          ,          l                    ·              w        m            
Third, the base station performs phase variation estimation using known pilot symbols inserted into a signal from the mobile station periodically at a phase variation estimating unit 106, so as to obtain a phase variation estimation value.
The base station performs a phase variation compensation on the combined signal Zk,l using the phase variation estimation value
      ξ    ↑        k    ,    l  obtained by the phase variation estimating unit 106 at a phase variation compensating unit 107, and performs a Rake combining. The signal on which the Rake combining is performed zk is represented by the following expression.
      z    k    =            ∑              l        =        1            L        ⁢                  z                  k          ,          l                    ·              ξ                  k          ,          l                *            
Fourth, the base station performs a phase identification and judgment process on the signal zk at a identifying and judging unit 108, and reproduces and outputs the transmitted data.
Then, the base station calculates an error vector using the above phase variation estimation value and an error signal at an estimated phase variation multiplying unit 109. The error signal is a difference between the signal on which the phase variation compensation is performed and the signal on which the identification and judgment process is performed.
A weighting factor controlling unit (MMSE) 104 updates the weighting factor w per symbol forming the signal from the mobile station, using the error vector and output signals from matched filters 1031 to 103M, so as to minimize the above error signal. A LMS (Least Mean Square) algorithm or RLS (Recursive Least Square) algorithm is used as an algorithm for updating the weighting factor w.
The base station compares a reference SIR with a received SIR calculated based on the above combined signal zk,l at an SIR measuring unit 110, so as to create a transmission power control signal which is to be transmitted to the mobile station.
As a result, the base station adopting the CAAAD receiving system allows the reduction of the interference signal power from other mobile stations, by forming and receiving directional beams each mobile station individually.
An adaptive antenna array transmission method is proposed in the technical literature “Adaptive antenna array transmit diversity in FDD forward link for WCDMA and broadband packet wireless access, IEEE Wireless Communications, pp.2–10, April. 2002. (H. Taoka, S. Tanaka, T. Ihara and M. Sawahashi)”. The adaptive antenna array transmission method compensates a beam pattern formed in an uplink for the amplitude variation and the phase variation which occur in radio transmission paths in a downlink, and transmits directional beams.
FIG. 2 shows an example of configurations of downlink channels in the DS-CDMA system using the above method.
As shown in FIG. 2, the downlink channels in the DS-CDMA system include a common pilot channel and an individual channel. The pilot symbols are time-division multiplexed in the whole of one slot in the common pilot channel. ND data symbols and NP pilot symbols are time-division multiplexed in one slot in the individual channel.
FIG. 3A shows a format of the individual channel for low speed in the DS-CDMA system, and FIG. 3B shows a format of the individual channel for high speed in the DS-CDMA system.
As shown in FIG. 3A, NL—D data symbols and NL—P pilot symbols are inserted into one slot in the individual channel for low speed periodically.
As shown in FIG. 3B, NH—D data symbols (NH—D>NL—D) and NH—P pilot symbols (NH—p>NL—P) are inserted into one slot in the individual channel for low speed periodically.
Generally, when the base station does not adopt the transmission of directional beams in the radio communication system, the mobile station performs channel estimation with the pilot symbols in the common pilot channel transmitted using non-directional beams, so as to compensate for the influence of the channel variation (the phase variation, the amplitude variation and so on) resulting from the fading which occurs in the radio transmission paths.
On the other hand, when the base station adopts the transmission of directional beams in the radio communication system, the base station forms directional beams in the direction of required signal, and transmits the individual channel using the directional beams.
As shown in FIG. 4, to be more specific, the base station 10 transmits the individual channel to the mobile station 30#1 via the antenna 101 using the transmission beam pattern for the mobile station 30#1 (a solid line), and transmits the individual channel to the mobile station 30#2 via the antenna 101 using the transmission beam pattern for the mobile station 30#2 (a dotted line).
In this case, power delay profiles in the radio transmission paths from the base station to the mobile station are different between the common pilot channel transmitted using non-directional beams and the individual channel transmitted using directional beams, and the pilot symbols inserted into the common pilot channel are not used in channel estimation, so that the channel estimation needs to be performed with the pilot symbols inserted into the individual channel.
However, there is a problem in that the power of the pilot symbols inserted into the individual channel is less than the power of the pilot symbols inserted into the common pilot channel, so that the accuracy of channel estimation is degraded, when the channel estimation with the pilot symbols inserted into the individual channel is performed in the conventional interference cancellation technique.
There is a problem in that the number of individual channels for low speed is less than the number of individual channels for high speed per one slot, and the power assigned to the pilot symbols inserted into the individual channel is low, so that the effect of averaging noise and interference signal power is little, the accuracy of channel estimation with the pilot symbols inserted into the individual channel is degraded, and downlink capacity is degraded.